Coal is the most abundant fossil fuel in the United States and comprises about 75% of the total resources of fossil fuels. However, this resource is not a good source of combustible fuel because of low-energy content, poor quality, and the presence of contaminants. It has been recognized that bioprocess technology has potential to convert this coal into an environmentally acceptable, energy-rich fuel with few contaminants. Liquefaction processes currently produce clean fuels from coal. However, these processes operate at high temperatures and pressures, making them unattractive. Conversely, bioprocessing of coal can produce clean fuels at mild temperatures and pressure which are not only safe, but may prove to be economical.
Subbituminous and lignite coals contain high levels of oxygen. A structural comparison of the coals is shown in Table 1.sup.1.
TABLE 1 ______________________________________ Structural Comparison of Coals Coal Carbon Nature of Nature of Rank Aromaticity Monomers Crosslinks ______________________________________ Lignite 30-50% Small, largely Many hydrogen single-single-ring bonds probably systems extensively some other cross- substituted with 0- links. Possibly salt functional groups bonds as in (--COOH, --OH, COO--Ca--OOC. --OCH.sub.3), about one Few aliphatic cross- oxygen per 3 to 4 links Gel-like; carbon. structural Water is important component. Sub- 60% Still mostly single Mixture of bitum- rings with some hydrogen bonds and inous larger rings. About probably ethers. one oxygen per 5 to Some aliphatic links. 6 carbon. ______________________________________
The key feature of the subbituminous coal is the presence of ether linkages, along with carboxyl groups, as predominant oxygen functional groups. This coal is highly reactive and not the refractory material it was once thought to be. However, under the severe processing conditions of temperature and pressure, the coal undergoes retrogressive condensation reactions resulting in an intractable coal. Thus, this type of coal is most suitable for biological processing, since these processes operate under mild conditions and can provide specific chemical transformations.
Prior art coal bioprocessing has been categorized into two areas. The first area is coal cleaning or the removal of undesirable components, such as sulfur, nitrogen, and trace metals. The second category is coal conversion, which includes microbial liquefaction, gasification, pretreatment, and methane production.
Physical cleaning is achieved by grinding (comminuting) of the coal to liberate impurities like mineral matter and ash that are not chemically bound and then taking advantage of specific gravity differences between the organic matter that formed the coals and the denser mineral impurities. Sometimes differences in surface wetting properties between the coal macerals and impurities are used for separation. The method of comminution generally involves mechanical comminution or grinding. In this method, the grinding is effected by ball or jet milling or any other techniques wherein the coal particles impinge against or are contacted with a solid obstruction. Jet milling, for example, involves entraining coal particles in a gas stream at a high velocity and directing the gas stream against a solid obstruction. Examples of jet milling are described in U.S. Pat. No. 3,897,010 (1975). Specific examples of such jet milling devices include the Micronizer brand fluid energy mill manufactured by Sturtevant Mill Co. and the "Jet-o-Mizer" fluid energy production mill produced by Energy Processing and Equipment Company.sup.3. Mechanical comminution techniques are frequently used to provide feed coal to a gasification reactor.
Ball milling, jet milling, and other mechanical impingement techniques involve relatively crude forms of comminution. First and most importantly, these techniques do not comminute selectively. That is, they grind both the ash forming minerals, as well as the carbonaceous fraction of the coal. Another disadvantage is that the mechanical grinding techniques do not separate or scission the carbonaceous matter within the coal from the mineral constituents of the coal. That is, ash forming materials generally remain physically attached to the carbonaceous material in the coal after milling to a considerable extent. The minerals thus cannot be removed from the desired carbonaceous fragment of coal. In addition, organic forms of sulfur remain chemically bonded in the hydrocarbon.
Another problem is that much of the energy in the grinding processes is lost or dissipated as heat energy and is not all used in the comminution of the coal particles. For example, the energy consumption for ultra-fine grinding of Illinois No. 6 coal to a particle size of 10 .mu. varies from 60 to 180 kwh per ton. This is a cost ineffective method.
Coal scientists have been trying to achieve an inexpensive approach to produce a decarboxylated, depolymerized, hydrogen-rich coal. Applicants' previous work has been directed at decarboxylating and reductively depolymerizing the coal under anaerobic conditions.sup.4,5. Applicants have continued to utilize anaerobic bacteria in fermentor systems and are applying anaerobic bioprocessing to covert a low-rank coal by decarboxylation and biodepolymerization to obtain a better fuel.
Physical coal cleaning is achieved by grinding the coal to liberate impurities that are not chemically bound and then taking advantage of specific gravity differences between the organic matter in coal (the macerals) and denser mineral impurities. It is recognized, however, that in the field of crushing and grinding (comminution):
Only several percent of the energy applied to the systems is actually used in fracturing the coal. The remainder is dissipated in process inefficiencies. PA1 Current techniques do not comminute selectively. Both the ash-forming minerals and the carbonaceous fraction of the coal are ground. This results in the fine mineral matter being intimately mixed and dispersed into the organic phase making separations difficult. PA1 The mechanical grinding techniques do not selectively separate or scission the carbonaceous matter within the coal from the mineral constituents of the coal. PA1 Current techniques are limited in the degree of size reduction.
Applicants' approach is based on bond cleavage under ambient conditions using microorganisms. It is appropriate to use a reductive approach, rather than an oxidative approach for removal of oxygen from low-rank coals. Oxygen from coal is removed by decarboxylation under reductive conditions in contrast to oxidation of coal under the latter approach.
The biodegradive potential of anaerobic bacteria has seen a tremendous expansion in the last 10-15 years. The prior art view that compounds not degradable aerobically will never be degraded anaerobically is no longer accepted. Most substrates in the presence of oxygen are attacked by oxygenases, which undergo basically different degradation reactions in the absence of oxygen.
Biological treatment with whole cells and isolated enzymes has a potential to yield useful products from low-ranked coals. The solubilization of coal by microorganisms was first reported by M. Cohen et al.sup.6.
While continuing the above-mentioned research, applicants discovered an unexpected reduction in coal particle size during anaerobic biotreatment. This "biogrinding" provides a process for reducing coal particle size which does not require elevated temperatures of the prior art or costly mechanical mechanisms for achieving the same results. Hence, the present invention provides a biotreatment for inexpensively and effectively decreasing coal particle size in a passive process.
Another unexpected observation is that the biotreatment modifies the coal such that the coal particles can remain suspended or dispersed in water for long periods without settling. This is an essential requirement/need for the utilization of coal-water slurries. Current coal-water slurries require the addition of expensive surfactants and other additives to achieve the dispersion of coal in water without settling. As stated earlier, this essential requirement must be met before coal-water slurries can be used.
Further advantage of the process is that only selective reduction in particle size of the carbonaceous component, and not of the mineral matter, in coal occurs during this biotreatment process in contrast to mechanical process where both carbonaceous and mineral matter components are grounded.